Fox Hunting

By david, on June 10th, 2013

This weekend I have been attending the SERG Ham radio convention in Mount Gambier where I have enjoyed the company of some good friends and learnt a lot about fox hunting.

A group of us decided to go 3 weeks ago. The well-connected Mark VK5QI has been scouring Adelaide for fox hunting equipment, including beam mounts, beams, Doppler arrays, sniffers, and mapping software. We spent the last few weekends setting up Andy VK5AKH’s car as our fox hunt vehicle.

I really enjoyed the mechanical side, for example I spent a pleasant Friday afternoon machining an alloy plate with an angle grinder and drill to fit the beam mount. Here I am test fitting on my EV:

My daughter reacted in horror when she saw this – “what have you done to my EV!” (yes – it’s “her” EV now since she started driving).

Three weeks is not enough time to get everything to work properly, but after a lot of hard work by Mark and Andy we were ready to compete in the sniffer hunts and 2m/70cm/23cm fox hunts.

Sniffer Hunt

The sniffer hunt required us to find 10 hidden transmitters on foot in the area around the Valley Lake. I partnered with Mandy VK5FMOO for the hunt:

I really enjoyed the sniffer hunt, especially the outdoor physical side for what can be an otherwise sedentary hobby. Reminded me of playing paint pall or orienteering. I found myself running up and down hills covered with scrub in order to get cross bearings, and getting stuck behind fences after a long walk only to find the wiley fox was on the other side.

I also learnt about radio propagation. Reflections (multipath) can give false bearings that lead you off in the wrong direction. Long distance bearings are usually more reliable. Foxes located high up can be heard a long way away, those at ground level can be hard to find until you are very close. The signals come and go as you move up and down on the terrain. High points in the terrain are a good spot for bearings. Alternate the antenna position between vertical and horizontal to get the best signal and bearing indication. Note the change in signal strength to determine range. Continuously re-tune as you may stumble across one fox while looking for another.

Mandy and I found 5/10 foxes which was a good first effort.

Fox Hunt

I also participated in the car based fox hunt with Andy and Mark. Here they are messing with a Doppler Array:

The fox would torment us by only transmitting for a few seconds every minute, this made it very hard to get a reliable bearing.

Some lessons learned:

A bad bearing at the beginning of the hunt can mean driving off in the wrong direction and losing the fox entirely. This was our fate for the first 2m leg and we had to admit defeat. Multiple bearings from a few different high positions at the beginning of the hunt is a good idea.

Like the sniffer hunts, long distance bearings are more reliable, as you get closer the situation gets confused by multipath. We spent an hour wandering around in circles in a clump of pine plantation:

All our bearings pointed into a 100 square metre chunk of this forest. We drove around and had multiple bearings into the forest. I thought we had that fox bracketed! So we hopped out with the sniffer but just got confusing, low level readings. We spent an hour in there. Eventually we found strong signal in another direction and finally found the fox several km away! It was very well obscured, the antenna taped to a small pine tree and the radio buried at the base of the tree:

The lessons I learned were – pine forest really confuses the bearings, use change in signal strength as an indication of range, and get more long distance bearings.